1. Field of the Invention
This invention relates to an implantable medical device that delivers sufficient electrical energy to cardiac tissue to terminate (cardiovert) tachycardias and thus restore normal sinus rhythm. An improved protection circuit prevents high voltage cardioversion pulses from damaging the sensing circuit of the device.
2. Description of the Prior Art
Implantable medical devices for the therapeutic stimulation of the heart are well known in the art from U.S. Pat. No. 3,478,746 issued to Wilson Greatbatch, which discloses a demand pacemaker. The demand pacemaker delivers electrical energy (5-25 microjoules) to the heart to initiate the depolarization of cardiac tissue. This stimulating regime is used to treat heart block by providing electrical stimulation in the absence of naturally occurring spontaneous cardiac depolarizations.
Another form of implantable medical device for the therapeutic stimulation of the heart is the automatic implantable defibrillator (AID) described in U.S. Pat. Nos. Re. 27,652 and Re. 27,757 to Mirowski, et al and the later U.S. Pat. No. 4,030,509 to Heilman et al. These AID devices deliver energy (40 joules) to the heart to interrupt ventricular fibrillation of the heart. In operation, the AID device detects the ventricular fibrillation and delivers a nonsynchronous high-voltage pulse to the heart through widely spaced electrodes located outside of the heart, thus mimicking transthoracic defibrillation. The Heilman et al technique requires both a limited thoracotomy to implant an electrode near the apical tip of the heart and a pervenous electrode system located in the superior vena cava of the heart. In practice, these devices have received limited usage due to the complexity of their implantation, their relatively large size and short operating life, and to the small numbers of patients who might benefit from it.
Another example of a prior art implantable cardioverter includes the device taught by U.S. patent application Ser. No. 58,847 to Engle, et al. This device detects the onset of tachyarrhythmia and includes means to monitor or detect the progression of the tachyarrhythmia so that progressively greater energy levels may be applied to the heart to interrupt the arrhythmia.
A further example is that of an external synchronized cardioverter, described in Clinical Application of Cardioversion in Cardiovascular Clinics, 1970,2, pp. 239-260 by Douglas P. Zipes. This external device is described in synchronism with ventricular depolarization to ensure that the cardioverting energy is not delivered during the vulnerable T-wave portion of the cardiac cycle.
Still another example of a prior art implantable cardioverter includes the device disclosed in U.S. Pat. No. 4,384,585 to Douglas P. Zipes. This device includes sensing circuitry to detect the intrinsic depolarizations of cardiac tissue and includes pulse generator circuitry to deliver moderate energy level stimuli (in the range of 0.1-10 joule) to the heart in synchrony with the detected cardiac activity.
The functional objective of this stimulating regime is to depolarize areas of the myocardium involved in the genesis and maintenance of re-entrant or automatic tachyarrhythmias at lower energy levels and with greater safety than is possible with nonsynchronous cardioversion. Nonsynchronous cardioversion always incurs the risk of precipitating ventricular fibrillation and sudden death. Synchronous cardioversion delivers the shock at a time when the bulk of cardiac tissue is already depolarized and is in a refractory state.
It is expected that the safety inherent in the use of lower energy levels, the reduced trauma to the myocardium, and the smaller size of the implanted device will expand the indications for use for this device beyond the patient base of prior art automatic implantable defibrillators. Since many episodes of ventricular fibrillation are preceded by ventricular (and in some cases, supraventricular) tachycardias, prompt termination of the tachycardia may prevent ventricular fibrillation.
Typically, the electrical energy to power an implantable cardiac pacemaker is supplied by a low voltage, low current, long-lived power source, such as a lithium iodine pacemaker battery of the types manufactured by Wilson Greatbatch Ltd. and Medtronic, Inc. While the energy density of these power sources is relatively high, they are not designed to be rapidly discharged at high current drains, as would be necessary to directly cardiovert the heart with cardioversion energies in the range of 0.1-10 joules. Higher energy density battery systems are known which can be more rapidly discharged, such as lithium thionyl chloride power sources. However, none of the available implantable, hermetically sealed power sources have the capacity to directly provide the cardioversion energy necessary to deliver an impulse of the aforesaid magnitude to the heart following the onset of tachyarrhythmia.
Generally speaking, it is necessary to employ a DC-DC converter to convert electrical energy from a low voltage, low current power supply to a high voltage energy level stored in a high energy storage capacitor. A typical form of DC-DC converter is commonly referred to as a flyback converter which employs a transformer having a primary winding in series with the primary power supply and a secondary winding in series with the high energy capacitor. An interrupting circuit or switch is placed in series with the primary coil and battery. Charging of the high energy capacitor is accomplished by inducing a voltage in the primary winding of the transformer creating a magnetic field in the secondary winding. When the current in the primary winding is interrupted, the collapsing field develops a current in the secondary winding which is applied to the high energy capacitor to charge it. The repeated interruption of the supply current charges the high energy capacitor to a desired level over time.
The delivery of the high energy cardioversion pulse is controlled by a memory and logic circuit which responds to a sensed tachycardiac according to programmable detection criteria. The pulse is delivered through an interface circuit which couples the pacing and cardioversion lead electrodes to the sensing circuit, the pacemaker output circuit and the high energy capacitor. The delivery of the high energy pulse creates a load on the sense amplifier due both to the energy of the pulse and the polarization after potentials that develop as the lead-tissue interface repolarizes following discharge.
The lead that is desirably employed in this system is described in U.S. Pat. No. 4,355,646. That lead has a large surface area anode electrode locatable in the superior vena cava and consisting of several ring electrodes electrically connected together. A ring and tip electrode are positioned at the distal portion of the lead adapted to be placed in the apex of the right ventricle. During cardioversion, the ring and tip electrodes are to be electrically connected together to form a large surface area cathode electrode. At other times the ring and tip electrodes are coupled to a sense amplifier or sensing circuit or pacing circuit. The present invention involves an improved circuit for effecting the electrical connection to avoid circuit damage and the elimination of polarization after potentials which interfere with sensing after cardioversion.